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Paring down pyroptosis: Shiga toxin is a phage-encoded exotoxin that interrupts protein translation and functions as a virulence factor for enterohemorrhagic Escherichia coli (EHEC), a human pathogen causing hemorrhagic colitis and acute renal failure. Havira et al. screened a panel of EHEC mutants lacking various virulence factors for mutants that interfered with inflammasome-mediated cell death. EHEC strains lacking Shiga toxin were more potent inducers of macrophage pyroptosis and IL-1β secretion than wild-type EHEC. Shiga toxin from wild-type EHEC interfered with pyroptosis by blocking the ability of the activated form of caspase-11, a cytoplasmic LPS sensor, to cleave gasdermin D and initiate formation of gasdermin pores in the plasma membrane. This unanticipated activity of Shiga toxin provides EHEC with an additional means of evading the innate immune system. Inflammatory caspase–dependent cytosolic lipopolysaccharide (LPS) sensing is a critical arm of host defense against bacteria. How pathogens overcome this pathway to establish infections is largely unknown. Enterohemorrhagic Escherichia coli (EHEC) is a clinically important human pathogen causing hemorrhagic colitis and hemolytic uremic syndrome. We found that a bacteriophage-encoded virulence factor of EHEC, Shiga toxin (Stx), suppresses caspase-11–mediated activation of the cytosolic LPS sensing pathway. Stx was essential and sufficient to inhibit pyroptosis and interleukin-1 (IL-1) responses elicited specifically by cytosolic LPS. The catalytic activity of Stx was necessary for suppression of inflammasome responses. Stx impairment of inflammasome responses to cytosolic LPS occurs at the level of gasdermin D activation. Stx also suppresses inflammasome responses in vivo after LPS challenge and bacterial infection. Overall, this study assigns a previously undescribed inflammasome-subversive function to a well-known bacterial toxin, Stx, and reveals a new phage protein-based pathogen blockade of cytosolic immune surveillance. [ABSTRACT FROM AUTHOR]

Nucleic acid sensing is a critical mechanism by which the immune system monitors for pathogen invasion. A set of germline‐encoded innate immune receptors detect microbial DNA in various compartments of the cell, such as endosomes, the cytosol, and the nucleus. Sensing of microbial DNA through these receptors stimulates, in most cases, interferon regulatory factor‐dependent type I IFN synthesis followed by JAK/STAT‐dependent interferon‐stimulated gene expression. In contrast, the detection of DNA in the cytosol by AIM2 assembles a macromolecular complex called the inflammasome, which unleashes the proteolytic activity of a cysteine protease caspase‐1. Caspase‐1 cleaves and activates the pro‐inflammatory cytokines such as IL‐1β and IL‐18 and a pore‐forming protein, gasdermin D, which triggers pyroptosis, an inflammatory form of cell death. Research over the past decade has revealed that AIM2 plays essential roles not only in host defense against pathogens but also in inflammatory diseases, autoimmunity, and cancer in inflammasome‐dependent and inflammasome‐independent manners. This review discusses the latest advancements in our understanding of AIM2 biology and its functions in health and disease. [ABSTRACT FROM AUTHOR]

Inflammasomes are cytosolic multi-molecular complexes that sense intracellular microbial danger signals and metabolic perturbations. Inflammasome activation leads to the activation of caspase-1 and the release of pro-inflammatory cytokines IL-1β and IL-18 accompanied by cell death. An inflammasome-based surveillance machinery for Gram-negative bacterial infections has been recently discovered. This noncanonical inflammasome relies on sensing the cytosolic presence of lipopolysaccharide of Gram-negative bacteria via inflammatory caspases such as caspase-4, -5, and -11. This review discusses the recent findings related to the mechanism of activation of the noncanonical inflammasome and its biological functions. [ABSTRACT FROM AUTHOR]

Summary Sensing of lipopolysaccharide (LPS) in the cytosol triggers caspase-11 activation and is central to host defense against Gram-negative bacterial infections and to the pathogenesis of sepsis. Most Gram-negative bacteria that activate caspase-11, however, are not cytosolic, and the mechanism by which LPS from these bacteria gains access to caspase-11 in the cytosol remains elusive. Here, we identify outer membrane vesicles (OMVs) produced by Gram-negative bacteria as a vehicle that delivers LPS into the cytosol triggering caspase-11-dependent effector responses in vitro and in vivo. OMVs are internalized via endocytosis, and LPS is released into the cytosol from early endosomes. The use of hypovesiculating bacterial mutants, compromised in their ability to generate OMVs, reveals the importance of OMVs in mediating the cytosolic localization of LPS. Collectively, these findings demonstrate a critical role for OMVs in enabling the cytosolic entry of LPS and, consequently, caspase-11 activation during Gram-negative bacterial infections. [ABSTRACT FROM AUTHOR]

Intracellular lipopolysaccharide (LPS) sensing by the noncanonical inflammasome comprising caspase-4 or -11 governs antibacterial host defense. How LPS gains intracellular access in vivo is largely unknown. Here, we show that CD14—an LPS-binding protein with a well-documented role in TLR4 activation—plays a vital role in intracellular LPS sensing in vivo. By generating Cd14 −/− and Casp11 −/− mice strains on a Tlr4 −/− background, we dissociate CD14's known role in TLR4 signaling from its role in caspase-11 activation and show a TLR4-independent role for CD14 in GSDMD activation, pyroptosis, alarmin release, and the lethality driven by cytosolic LPS. Mechanistically, CD14 enables caspase-11 activation by mediating cytosolic localization of LPS in a TLR4-independent manner. Overall, our findings attribute a critical role for CD14 in noncanonical inflammasome sensing of LPS in vivo and establish—together with previous literature—CD14 as an essential proximal component of both TLR4-based extracellular and caspase-11-based intracellular LPS surveillance. [Display omitted] • CD14 is vital for cytosolic LPS sensing by the noncanonical inflammasome in vivo • CD14 enables caspase-11 activation by facilitating the cytosolic transfer of LPS • The role of CD14 in the cytosolic sensing of LPS is TLR4-independent How LPS attains cytosolic access in vivo is unclear. Vasudevan et al. define a TLR4-independent role for CD14 in the cytosolic localization of LPS, triggering noncanonical inflammasome activation and pyroptosis in vivo. This finding positions CD14 as an integral component of both extracellular and intracellular LPS surveillance pathways. [ABSTRACT FROM AUTHOR]

Caspase-11 sensing of intracellular lipopolysaccharide (LPS) plays critical roles during infections and sepsis. However, the key cell types that sense intracellular LPS and their contributions to the host responses at the organismal level are not completely clear. Here, we show that macrophage/monocyte-specific caspase-11 plays a dominant role in mediating the pathological manifestations of endotoxemia, including gasdermin D (GSDMD) activation, interleukin (IL)-1β, IL-18, and damage-associated molecular pattern (DAMP) release, tissue damage, and death. Surprisingly, caspase-11 expression in CD11c+ cells and intestinal epithelial cells (IECs) plays minor detrimental roles in LPS shock. In contrast, caspase-11 expression in neutrophils is dispensable for LPS-induced lethality. Importantly, caspase-11 sensing of intracellular LPS in LyzM+ myeloid cells and MRP8+ neutrophils, but not CD11c+ cells and IECs, is necessary for bacterial clearance and host survival during intracellular bacterial infection. Thus, we reveal hierarchical cell-type-specific roles of caspase-11 that govern the host-protective and host-detrimental functions of the cytosolic LPS surveillance. [Display omitted] • Macrophage/monocyte-specific caspase-11 plays a dominant detrimental role in sepsis • DC- and intestinal epithelial cell-specific caspase-11 play minor roles in sepsis • Neutrophil-specific caspase-11 is dispensable for lethal LPS shock • Macrophage- and neutrophil-specific caspase-11 confer antibacterial defense Kumari et al. reveal hierarchical cell-type-specific roles of caspase-11 that govern the host-protective and host-detrimental functions of the cytosolic LPS surveillance pathway during bacterial infections and sepsis, respectively. [ABSTRACT FROM AUTHOR]

Summary Inflammasome-activated caspase-1 cleaves gasdermin D to unmask its pore-forming activity, the predominant consequence of which is pyroptosis. Here, we report an additional biological role for gasdermin D in limiting cytosolic DNA surveillance. Cytosolic DNA is sensed by Aim2 and cyclic GMP-AMP synthase (cGAS) leading to inflammasome and type I interferon responses, respectively. We found that gasdermin D activated by the Aim2 inflammasome suppressed cGAS-driven type I interferon response to cytosolic DNA and Francisella novicida in macrophages. Similarly, interferon-β (IFN-β) response to F. novicida infection was elevated in gasdermin D-deficient mice. Gasdermin D-mediated negative regulation of IFN-β occurred in a pyroptosis-, interleukin-1 (IL-1)-, and IL-18-independent manner. Mechanistically, gasdermin D depleted intracellular potassium (K+) via membrane pores, and this K+ efflux was necessary and sufficient to inhibit cGAS-dependent IFN-β response. Thus, our findings have uncovered an additional interferon regulatory module involving gasdermin D and K+ efflux. Graphical Abstract Highlights • Inflammasome-activated gasdermin D limits type I interferon responses to cytosolic DNA • Gasdermin D targets cGAS activation to inhibit IFN-β response to cytosolic DNA • Depletion of intracellular K+ by gasdermin D is responsible for limiting cGAS signaling • K+ efflux is sufficient to inhibit cGAS-dependent type I interferon responses Gasdermin D is a pore-forming protein, which upon activation by inflammasome complexes mediates pyroptotic cell death and IL-1 release. Banerjee et al. demonstrate a previously unknown regulatory role for gasdermin D-driven K+ efflux in reining in cGAS-dependent type I interferon response to cytosolic DNA. [ABSTRACT FROM AUTHOR]

Glutathione peroxidase 4 (GPX4) is an antioxidant enzyme that protects cells from lipid peroxidation. In this issue of Cell Host & Microbe, Kang et al. (2018) report that GPX4 is a negative regulator of the pyroptotic cell death pathway and plays an important role in inhibiting lethal inflammation associated with sepsis., (Copyright © 2018 Elsevier Inc. All rights reserved.)

Actin nucleation factors function to organize, shape, and move membrane-bound organelles, yet they remain poorly defined in relation to disease. Galloway-Mowat syndrome (GMS) is an inherited disorder characterized by microcephaly and nephrosis resulting from mutations in the WDR73 gene. This core clinical phenotype appears frequently in the Amish, where virtually all affected individuals harbor homozygous founder mutations in WDR73 as well as the closely linked WHAMM gene, which encodes a nucleation factor. Here we show that patient cells with both mutations exhibit cytoskeletal irregularities and severe defects in autophagy. Reintroduction of wild-type WHAMM restored autophagosomal biogenesis to patient cells, while inactivation of WHAMM in healthy cell lines inhibited lipidation of the autophagosomal protein LC3 and clearance of ubiquitinated protein aggregates. Normal WHAMM function involved binding to the phospholipid PI(3)P and promoting actin nucleation at nascent autophagosomes. These results reveal a cytoskeletal pathway controlling autophagosomal remodeling and illustrate several molecular processes that are perturbed in Amish GMS patients., (© 2017 Mathiowetz, Baple, et al. This article is distributed by The American Society for Cell Biology under license from the author(s). Two months after publication it is available to the public under an Attribution–Noncommercial–Share Alike 3.0 Unported Creative Commons License (http://creativecommons.org/licenses/by-nc-sa/3.0).)

Small G-proteins are key regulatory molecules that activate the actin nucleation machinery to drive cytoskeletal rearrangements during plasma membrane remodeling. However, the ability of small G-proteins to interact with nucleation factors on internal membranes to control trafficking processes has not been well characterized. Here we investigated roles for members of the Rho, Arf, and Rab G-protein families in regulating WASP homologue associated with actin, membranes, and microtubules (WHAMM), an activator of Arp2/3 complex-mediated actin nucleation. We found that Rab1 stimulated the formation and elongation of WHAMM-associated membrane tubules in cells. Active Rab1 recruited WHAMM to dynamic tubulovesicular structures in fibroblasts, and an active prenylated version of Rab1 bound directly to an N-terminal domain of WHAMM in vitro. In contrast to other G-protein-nucleation factor interactions, Rab1 binding inhibited WHAMM-mediated actin assembly. This ability of Rab1 to regulate WHAMM and the Arp2/3 complex represents a distinct strategy for membrane remodeling in which a Rab G-protein recruits the actin nucleation machinery but dampens its activity., (© 2016 Russo et al. This article is distributed by The American Society for Cell Biology under license from the author(s). Two months after publication it is available to the public under an Attribution–Noncommercial–Share Alike 3.0 Unported Creative Commons License (http://creativecommons.org/licenses/by-nc-sa/3.0).)